FIGS. 13, 14, 15A and 15B are views for explaining a driving force transmission device for transmitting a driving force to a photoconductor drum according to the background art as disclosed in JP-A-8-270642. FIG. 13 is a partial perspective view of the driving force transmission device. FIG. 14 is a schematic configuration view of the driving force transmission device. FIGS. 15A and 15B are explanatory views of a coupling unit in the driving force transmission device.
In the driving force transmission device, a cartridge-side coupling unit is provided in one end portion of a photoconductor drum 201. In the coupling unit, a coupling recessed shaft 203 is provided in a flange 202 fixedly attached to the end portion of the photoconductor drum 201. A recess 204 is formed in a central portion of an end surface of the recessed shaft 203.
On the other hand, a body-side coupling unit is provided on the body side of the image forming apparatus. In the coupling unit, a coupling protruding shaft 205 is provided in a position corresponding to the rotation axis of the photoconductor drum 201. As shown in FIG. 14, torque is transmitted from a pinion gear 207 of a motor 206 to the protruding shaft 205 through a gear 208. A protrusion 209 is provided in a tip portion of the protruding shaft 205.
The gear 208 is fixedly attached to the coupling protruding shaft 205. The gear 208 and the pinion gear 207 constitute a helical gear. The helical gear has a gear configuration in which thrust in a direction of the arrow a of FIG. 14 can be generated in the gear 208 when the coupling protruding shaft 205 is rotated in a direction in which the photoconductor drum 201 should rotate to form an image thereon. By this thrust, the protrusion 209 provided in the tip portion of the protruding shaft 205 is inserted into the recess 204 provided on the photoconductor drum 201 side. Thus, the recessed shaft 203 and the protruding shaft 205 engage with each other.
As shown in FIGS. 14 and 15A, the protrusion 209 is shaped into an equilateral triangle pole, and the recess 204 has a shape bored in an equilateral triangle pole which is large enough to insert the protrusion 209 thereto.
The cross-sectional triangle of the protrusion 209 and the cross-sectional triangle of the recess 204 have a relationship as shown in FIG. 15B. That is, the triangles satisfy the relation of D1<D0<D2 when D0 designates the diameter of a circumscribed circle R0 of the triangle of the protrusion 209, D1 designates the diameter of an inscribed circle R1 of the triangle of the recess 204, and D2 designates the diameter of a circumscribed circle R2 of the triangle of the recess 204.
Patent Document 1: JP-A-8-270642
In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a photoconductor drum by laser scanning, and toner is applied to the latent image by a developing unit so as to form a toner image. The toner image is transferred to an intermediate transfer belt by a first transfer unit. The toner image is transferred from the intermediate transfer belt to paper by a second transfer unit. In such an electrophotographic image forming apparatus, it is necessary to drive and rotate the photoconductor drum at a constant angular velocity. When the angular velocity of the photoconductor drum changes, the position where the image will be exposed on the photoconductor will be shifted. Thus, unevenness will appear in density if the image is monochrome. If the image is a full-color image, misalignment will appear when a plurality of colors are superimposed on the intermediate transfer belt. Thus, color shift will occur.
The photoconductor drum deteriorates with age in its electrostatic characteristic, which will affect abrasion or exposure of its photosensitive layer. Therefore, the photoconductor drum is generally set as a replaceable part. The photoconductor drum is removably supported on an image forming apparatus body.
In view of cost and mounting accuracy, a driving system including a driving shaft, a motor, etc. for the photoconductor drum is not made replaceable but fixed to the image forming apparatus body. An encoder can be attached to detect the angular velocity of the driving shaft. Thus, the driving shaft can be controlled to rotate at a constant angular velocity. However, the driving force transmission device may have looseness. Even if the driving shaft rotates at a constant angular velocity, fluctuation may occur in the angular velocity of the photoconductor drum due to the looseness of the driving force transmission device. Thus, the aforementioned image deterioration such as density unevenness or color shift may occur.
The aforementioned background-art driving force transmission device shown in FIGS. 13, 14, 15A and 15B has a shape with looseness where the recess 204 and the protrusion 209 can come in contact at three points. The recess 204 and the protrusion 209 are pressed on each other by a driving force so that driving and transmission can be stabilized. However, the driving force transmission device configured thus may be affected by an external force applied perpendicularly to its axis. In such a case, the contact cannot be stabilized. The angular velocity which should be transmitted to the photoconductor drum 201 may be fluctuated. Thus, there is a problem in operational reliability as a driving force transmission device.